1. Field of the Invention
The present invention relates to a variable gain amplifier circuit.
2. Description of the Related Art
In an integrated circuit such as an optical pickup IC, a variable gain amplifier circuit which amplifies an input signal with a desired gain may be used. FIG. 2 depicts a configuration of a typical variable gain amplifier circuit (hereinafter, VGA: Variable Gain Amplifier) 100 (see, e.g., Japanese Laid-Open Patent Publication No. 2008-301035). The VGA 100 includes an operational amplifier 110, resistors 120 to 122, switches 130, 131, and a capacitor 140. It is assumed that the resistance values of the resistors 120 to 122 are R0 to R2, respectively. For example, when only the switch 130 is turned on, the resistor 121 is connected between an inverting input terminal and an output terminal of the operational amplifier 110. Therefore, the magnitude of the direct-current gain of the VGA 100 is R1/R0. For example, when only the switch 131 is turned on, the magnitude of the direct-current gain of the VGA 100 is R2/R0. As such, ON/OFF of the switches 130, 131 is controlled, to change the magnitude of the direct-current gain of the VGA 100. The capacitor 140 connected between the inverting input terminal and the output terminal of the operational amplifier 110 has a capacitance which limits the frequency band of the VGA 100.
The resistors 130, 131 are feedback resistors and have parasitic capacitances. Therefore, if the switches 130, 131 are switched ON/OFF, a capacitance value of a feedback loop of the operational amplifier 110 may be changed and a phase margin of the operational amplifier 110 may deteriorate. If the phase margin of the operational amplifier 110 deteriorates, the VGA 100 may oscillate.